Various systems use transponders to identify objects from a distance by associating a transponder with the object. The transponders are programmed with unique identification codes that identify the object. Typically, an interrogator transmits an interrogation signal. When a transponder receives the interrogation signal, it responds by broadcasting its unique identification code. The interrogator identifies the transponder and the object with which it is associated by detecting this identification code.
The high cost, excessive bulk and poor reliability of conventional transponders prevent their use in numerous applications. A highly reliable, small transponder which may be provided at such a low cost that it may be regarded as a simple "tag" is needed to permit these and many other applications to identify objects from a distance. These applications may include, for example, tags for identifying luggage, employees, vehicles, and goods in inventory.
While transponders that include a power source have improved range, these transponders are more expensive and less reliable than passive transponders. Passive transponders obtain the energy required for their operation from the interrogation signal itself For example, the response signal may be broadcast by modulating the reflectivity of the transponder to the interrogation signal rather than by actively sourcing and radiating an RF signal.
Such conventional transponders use an antenna printed on one portion of the circuit board and mount various discrete, individually packaged electrical components, such as semiconductor chips, diodes, resistors, capacitors, inductors and the like on remaining portions of the circuit board. The antenna size is limited by the available circuit board area which must be shared with the electrical components. An efficient antenna must have dimensions that are determined by the frequency of the interrogation signal. To provide a compact transponder, the transponder must operate at very high frequencies, typically in the GHz region. Individual components are poorly suited for operation at these high frequencies.
As noted above, each tag must respond with a response signal that identifies the tag. In some applications, a number of identical tags may be deployed; in other applications, each tag must have a unique response signal. Accordingly, it would be advantageous to be able to provide a single tag design that can be easily programmed with its identification code.
Finally, as noted above, the cost of manufacturing tags has limited the use of radio tags. It would be advantageous to be able to manufacture radio tags as single chips using conventional semiconductor manufacturing techniques.
Broadly, it is the object of the present invention to provide an improved radio tag.
It is a further object of the present invention to provide a radio tag that operates at high frequencies.
It is yet another object of the present invention to provide a radio tag that does not require discrete components.
It is a still further object of the present invention to provide an inexpensive method for making a radio tag.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.